Circuit arrangement for automatic telephone installations



March 17,, 1942. E, z Rzow Em 2,276,447

CIRCUIT ARRANGEMENT FOR AUTOMATIC TELEPHONE INSTALLATIONS Filed Aug. 25, 1959 2 Sheets-Sheet 1 E F H3 1 (1 IM 7 1 U6! 1 12M N u 1 3501 I II BY @Z ATTOR/VEK March 17, 1942. E, ZIRZOW HAL. 2,276,447

CIRCUIT ARRANGEMENT FOR AUTOMATIC TELEPHONE INSTALLATIONS Filed Aug. 25, 1959 I 2 Sheets-Sheet 2 FIG 3 Ue2 :EWi2 /l01v2 102v? E INVENTORS: E 60 Z lRZOW zwmsmr TOPFER B) Z M 770R/VE Patented Mar. 17, 1942 UNITED STATES PATENT OFFICE CIRCUIT ARRANGEMENT FOR AUTOMATIC TELEPHONE INSTALLATIONS corporation of Germany Application August 25, 1939, Serial No. 291,810 In Germany August 30, 1938 4 Claims. (Cl. 179-1'6) The invention relates to means for insuring the release of an interexchange connection at any time during the establishment of such connection when the calling party desires such release. When an interexchange trunk line conductively blocked by repeating coils is used for such connection it is sometimes difficult to prevent interference between signals transmitted in one direction for release and signals transmitted in the other direction for metering and the like. In order to insure release when a calling subscriber hangs up even though this occurs before the called subscriber has answered, after he has answered or at the same time, We have provided an arrangement for sending a single inductively transmitted impulse when release is desired before the called party has answered and by means of a series of short successive inductive impulses after the called party has answered.

In the present system we have shown an interexchange trunk line VL terminating in an outgoing repeater Uel (Fig. 1) in one exchange and an incoming repeater U62 (Fig. 3) in the called or terminating exchange. Each repeater is connected to the trunk line VL by means of a repeating coil.

When the outgoing end of repeater Uel is seized to extend a connection over the trunk line or junction VL through repeater Ue2 (Fig. 3) the equipment in Uel is operated to send over the trunk line positive and negative induced impulses for operating the usual switches to extend the connection. The signal returned when the called subscriber responds by lifting his receiver is sent back over the trunk line VL by means of alternating current. The release before the response of the called party is accomplished by a single induced impulse, while after the called party has answered and should the calling party wish to release then a series of short successive induced pulses are transmitted.

The accompanying figures show two embodiments of the invention. Figures 1 and 3 show an embodiment in which all the switch operations are transmitted from the outgoing to the incoming end of the junction by induced impulses. In particular the release signal, transmitted when the calling subscriber hangs up his receiver after the signal for the conversation to begin, which occurs on the reply of the called subscriber, is sent by means of an odd number of alternations of positive and negative inductive impulses separated by short pauses.

On the other hand Figure 2 shows an embodiment in which the release impulse, transmitted after the called subscriber has replied is accomplished not by means of inductive impulses transmitted when the calling subscriber hangs up his receiver but by alternating current impulses.

Inthe reverse direction signals are transmitted in both embodiments by alternating current impulses. Instead of the alternating current impulses it is understood that signals in the reverse direction can also be transmitted as induced impulses without essentially altering the natur of the invention in any way.

In order to ensure that the release impulses get through, their frequency is made higher than the frequency of the impulses proceeding in the reverse direction. Moreover the duration of the release impulse is at least longer than lperiod of the impulses in the reverse direction.

The embodiment shows an arrangement with a trunk line arranged for uni-directional trafiic.

The invention it is understood, can also be used for trafilc in two directions.

The method of working of the arrangement in Figs. 1 and 2 is as follows:

Seizure On the seizing of the repeater Uel (Fig. 1) on the outgoing end of the junction VL by means of a group selector (not shown) the relay Cl is energised over the c-conductor: earth, test relay, test wiper of the series group selector, conductor 0, contacts Haul, l-2hl, l3bl, l5cl, winding II of relay Cl, battery, earth. Relay Cl, by opening its contact l5cl and closing its contact Mel, connects its high resistance winding I in'in series with winding II in the seizing circuit and holds over this contact independently of relays XI, HI and Bi. Relay Cl closes the following circuit for relay Al at contacts 260! earth, contact 2l2'l, winding II of relay Al, contact l9atl, winding III of relay 'CI, contact 2601, battery, earth. Moreover, before relay Al is energized, relay CI closes the following circuit over the winding I of the transformer Tr: earth, contacts .lbl, 31ml, 32cl, winding I of transformers Tr, battery, earth. Thus in the winding II of transformer Tr, an impulse is induced which however is without efiect since the contact 35ml is not yet closed. Relay AI closes the following circuit for relay BI earth, contacts 2lil, llal, winding I of relay Bl, Winding I of relay VI, contact 26cl, battery, earth. Relay Bl operates in series with relay VI. Relay Bl at contact 912! interrupts the circuit of the wind ng I of transformer Tr. Thus in the winding II of the transformer Tr an inductive impulse is produced whose direction is indicated .as negative. This impulse, which extends over contacts 35aI, 3IeI, 38vI and after the operation of the relay VI directly over contact 39vl and the primary winding of the transformer blocking the line, is transmitted over the junction VL to the incoming repeater Ue2 (Fig. 3). The polarisation of the accepting relay P2 (Fig 3) shunting the line, is arranged so that this impulse is without effect on the armature of the relay. The impulse furthermore serves to effect a complete demagnetisation of the relay P2 so far as any residual field is concerned. Relay BI (Fig. 1) closes the following circuit for relay XI: earth, contact 2 lil, winding I of relay XI, contacts I8bI, 26cI, battery, earth. Relay XI operates and closes a further circuit over the winding I of transformer Tr: earth, contacts IObI, 3I:rI, 32cI, winding I of transformer Tr, battery, earth. By the closing of this circuit an inductive impulse in the positive direction is produced in the Winding II of the transformer Tr which extends over the junction VL to the polarised accepting relay P2 in the incoming repeater Ue2. Thus as is shown later the repeater Ue2 is seized. In the outgoing repeater UeI after the energising of relay XI at contact I9:cI the circuit for the winding II of relay AI is interrupted and this relay restores. By the release of relay AI at contact I'IaI the circuits for relays BI and VI are opened. Relays BI and VI restore and the latter in View of the short circuit over contact 29yl for its winding II has a delayed release. By the release of relay BI at contact IObI the circuit over the winding I of transformer Tr is interrupted. By the opening of this circuit, in the winding II of the transformer Tr, a negative inductive impulse is produced which, since the contact 39vI is still closed, extends over the junction VL to the polarised accepting relay P2 of the repeater Ue2. The armature of the relay P2 is thus set back to its initial position. In the outgoing repeater UeI after the release of relay BI the circuit for the winding I of relay XI at contact I 8bI is interrupted. The relay holds in the following circuit however: earth, contacts 46ml, 5IIhI and 48aI,

winding II of relay XI, battery, earth.

In the incoming repeater Ue2 as already mentioned by means of the positive seizing impulse, which extends over the winding I of relay P2 shunted across the line, the seizing operation is initiated. It should first be stated that the relay E2 is energised in the normal condition of the installation over its high resistance winding I and the c-conductor to the following group selector. Thus over the closed contacts 6Ie2 and 64e2 the circuit for the accepting relay P2 is completed. The relay P2 after its operation closes the following circuit for relay A2: earth, contacts 90112, 18]2, winding of relay A2, battery, earth. Relay A2 operates and closes the following circuit for the seizing relay C2: earth, winding of relay C2, contacts Ill3a2, IOIv2, resistance Wi2, battery, earth. Relay C2 operates and holds independently of relays A2 and V2 in the following circuit: earth, winding of relay C2, contacts 99f2, III'Ic2, resistance Wi2, battery, earth. Further, relay C2 by direct earthing of the c-conductor over contact I602 seizes the connected group selector in known manner.

operates. The seizing relay G2 which is now short-circuited over contacts I 0202, I03a2 is however not restored, since by means of the negative inductive impulse arriving after the short interval relay P2 and hence also relay A2 are restored before relay C2 releases. The relay V2 is then short-circuited over contact 8Ia2 and restores. After the release of relay A2 the circuit for relay U2 is completed: earth, contacts 8Is2, 8602, 8Ia2, winding II of relay U2, contacts 88a2, 83171.2, resistance Wi3, battery, earth. Over its contact 84u2 relay U2 prepares a holding circuit including the two windings I and II in series which circuit is independent of relay A2. Further by closing its contact 9 Iu2 relay U2 prepares a circuit for its winding III.

Transmission of operating impulses By means of the numerical selection in the group selector (not shown) impulses received from the repeater UeI, earth is connected intermittently to the a-conductor (Fig. 1) and thus the Winding I of the relay AI of the repeater UeI is intermittently energised: earth in the group selector, conductor a, contacts IhI, 56M, winding I of relay AI contacts 3yI, Eel, battery, earth. Relay AI' operates and over contact I'IaI energises relays BI and VI. Relay VI holds as a slow relay during the impulse train. Relay BI on each operation closes over contact IiIbI a circuit over the winding I of the transformer Tr and thus produces a positive inductive impulse over the junction VL. When the relay AI restores the circuit for relay BI is interrupted and relay BI restores interrupting the circuit over winding I of the transformer Tr. Thus a negative inductive impulse is transmitted over the junction. In this manner in the numerical selection alternative positive and negative inductive impulses are transmitted over the junction.

In the incoming repeater Ue2 during the numerical selection the armature of the relay P2 is operated and then shortly afterwards restored by means of a pair of inductive impulses, one in the positive and one in the negative directions; thus the contact 90122 is momentarily closed and opened. By the closing of contact p2 relay A2 is energised. Relay A2 interrupts the short circuit for the slow relay V2 at contact 8'Ia2 so that this also operates. Relay V2 holds during the impulse train. Relay V2 is maintained energised in the following circuit when contact 88112 is opened during the impulse transmission: earth, contacts 8Is2, 8602, winding I of relay V2, contact 89m2, windings II and I of relay U2, contacts 84u2, 83m2, resistance Wi3, battery, earth. The impulses are transmitted by means of relay A2 in the following circuit to the connected group selector: earth, contacts IIIuZ, 68a2, 6902, conductor a, impulse accepting relay of the group selector which follows, battery, earth. During the impulse train control potential is connected to the b-conductor in the following circuit: earth, battery, contacts 'I5m2, IdsZ, 12122, conductor b. The seizing relay C2 short-circuited over contact I 02122 and the intermittently closed contact I03a2 during the impulse train is 'not restored during a single impulse train. Moreover during an impulse train the a-conductor is interrupted by means of contact 65112 in the direction opposite to the blocking transformer so that any undesired charging up of the talking condenser by the impulse transmission may be avoided.

Release control The case is now considered in which the calling subscriber hangs up during the impulse selection before the called subscriber has replied and thus initiates the release. In the group selector which has seized the outgoing repeater Uel the c-conductor is thus interrupted. The relay Ci of the repeater Uel is restored. Thus relay Al is energised again in the following circuit: earth, contact 2lz'l, winding II of relay Al, contacts 20rd, 23yl, Winding II of relay I-II, contacts 25:11, 2'lcl, battery, earth. Before relay Hi which is slow to operate has energised in this circuit its winding II is again short-circuited over contact Zeal. By the opening of contact 25a! the short circuit for winding I of relay El is interrupted. Rela El slowly operates in series with the winding II of relay Al. On the operation of relay Ai the folowing circuit is completed over the winding I of transformer Tr: earth, contacts 48a], 330i, windin I of transformer battery, earth. Thus a positive impulse is induced in the winding II of the transformer Tr which as long as the relay El has not yet operated is transmitted over this transformwinding and over the contacts 35M, Slel, 381;! to the blocking transformer and from this over the junction VL to the incoming repeater Ue2. The negative inductive impulse which follows is suppressed by the operation of relay El which interrupts the repeater circuit at contact 3lel. As soon as relay E! has operated the following circuit is completed for relay Bl: earth, contacts 25ml, 49m, 4322!, lZeL fiEcl, winding II of relay Bl, battery, earth. Relay BI by means of its contact 5519i short-circuits the holding circuit for the winding II of relay Xl. Relay Xi thus restores and interrupts the circuit for the winding II of relay Bl which restores. By the release of relay XI the circuit for relays Al and El is also interrupted at contact 29ml and both relays restore. Thus all the relays for the repeater Uel are in their normal positions.

In the repeater Ue2 by means of the positive release inductive impulse over winding I the accepting relay P2 is energized. Contact 98102 closes a longer energising circuit for relay A2. Relay A2 interrupts at contacts 8M2 the shortcircuit for relay V2 and relay V2 also operates. The seizing relay C2 is now short-circuited over contacts 32272 and lil3a2 so that it restores. By the release of the relay C2 the seizing of the cconductor as far as the group selector connected with contact "I602 is discontinued and its release initiated. After the release of relay C2 by means of the opening of contact 8602 the circuit for relays V2 and E2 is interrupted and also the circuit for relay U2. The relays restore. Thus the following circuit is completed for relay F2: earth, winding II f relay F2, contacts 9102, ll8e2, battery, earth. Relay F2 operates and closes the following circuit for winding II of relay P2: earth, contacts 9326!, 92f2, winding II of relay P2, battery, earth. Over this winding which is a reverse winding to the winding I the armature of the relay P2 is again set back to normal so that relay A2 restores. As soon as the group selector seized by repeater Ue2 has been released the resistance winding I of relay E2 is operated again in known manner over the principal contact of this selector and on the one hand at contact 83e2 interrupts the circuit for relay F2 and on the other hand again disconnects the blocking alternating current connected during the release of relay E2 over contacts 62e2 and 63e2 back over the junction.

In the repeater Uel by means of the blocking alternating current transmitted revertively over the junction VL the relay J l is energised in the following circuit: repeater windings, contacts 3322i, 35el, winding of relay Jl, repeater winding. The relay J l closes the following circuit for the relay HI: earth, contact 2221, Winding III of relay I-II, earth. Relay HI by opening contact l2h! interrupts the incoming c-conductor and thus prevents the premature seizing of the repeater Uel. As soon as the release has been accomplished at the incoming end of the line the relays J i and Hi restore and the guarding is discontinued. 1

Response of called subscriber The case is now considered in which the release is initiated after the called subscriber has replied. When the receiver of the called subscriber is lifted metering potential is connected to the b-conductor from the final selector onwards. In the incoming repeater Ue2 the relay S2 thus operates-in the following circuit: potential on the b-conoluctor, contact I ['02, winding II of relay S2, contacts 6lm2, 10102,. earth. Relay S2 at contact 8582 interrupts the holding circuit for winding II of relay E2 which restores. Relay U2 Whose energising circuit is also interrupted by opening of contact 8ls2 is maintained energised in the following circuit: earth, contacts 8M2, 95.92, winding III of relay U2, battery, earth. Relay E2 over its contact 62:22 and 63e2 momentarily connects alternating current potential WQ back over the junction VL. The connection of alternating current to the junction is terminated again when after the release of relay E2 relay V2 is energised in the following circuit: earth, contact H3402, Winding II of relay V2, contact 9502, 9862, battery, earth. Relay V2 operates and at contact 102122 interrupts a short circuit for the winding I of relay M2 so that this relay operates in series with relay V2. Relay M2 holds in the following circuit: earth, winding II of relay contact l32m2, resistance Wz'3,

battery, earth. Now' the circuit for winding II of the relay S2 which after the energisation of relay V2 was maintained over the b-conductor over contacts 12112, 1332, is interrupted at contact 6lm2. Over the closed contact 551,22 both wind: ings II- and I are connected in series across the talking conductor in order to serve as terminating signal relays at the end of a connection. -Relay S2 restores. Thus relay E2 is again energised over its winding II and by opening contact B2e2 and E3e2 disconnects alternating current source WQ from the junction again and on the other hand by means of contact file2 and 6492 switches throughv the two talking conductors again. At the release of relay S2 relay U2 is again held over its winding II. Relay V2 whose circuit'is-interrupted at contact 98e2 is restored and switchesv through the two talking conductors again at contacts 65272 and H122.

In the outgoing repeater Uel on the reply of circuit: earth, contact 46ml, winding II of relay El, contact 52yI, winding of relay YI, resistance Wil, battery, earth. In this circuit relay El also operates. By means of contact 3yI the winding I of the impulse accepting relay AI is disconnected from the incoming a-conductor. Over contact Bel and the choke Ddb metering potential is connected to the b-conductor. Over contact IeI and choke Dra the transmission of the terminating signal is prepared.

Release after called subscribers response When at the end of the conversation the calling subscriber hangs up then in the outgoing repeater UeI the seizing relay CI is again restored. Thus relay AI is energised in the following circuit: earth, contact 2IiI, winding II of relay AI, contact 20ml, windings I and II of relay HI, contacts 25al, 2'lcI, battery, earth. Relay AI closes its contact MIaI whereby a circuit is completed over winding I of transformer Tr which results in the transmission of a positive inductive impulse over the junction. In series with the relay AI relay HI also energizes but only after a delay. The energising of relay El over its winding I in series with the relay HI and relay AI is without effect since relay EI has already been energised over its winding II. Relay VI is energised before ,the relay HI in the following circuit: earth winding II of relay VI, contacts 28yl, 2'Icl, battery, earth. Relay VI over its contact Ifl8vl short-circuits relay Y I so that this restores slowly. Relay YI on the one hand over its contact 29yI short-circuits the winding II of relay VI so that this relay also restores slowly. During the release period of relay VI a number of inductive impulses in alternate directions are transmitted over the junction. This happens in that after the operation of relays AI and HI in series with winding II of relay Al the holding circuit for winding II of relay XI is interrupted at the contacts flhl, and 48ml. Relay XI restores and interrupts the circuit for AI at contact, 20ml. The relay AI restores while relay HI is still held. Through the release of relay Al the circuit over winding I of transformer Tr is interrupted at contact 4lJaI and simultaneously over contact Mal a further energising circuit is completed for relay XI: earth, contacts IIaI, 4I4vI, winding II of relay XI, battery, earth. Relay XI operates again and over contact 20ml again connects the winding II of relay AI in the already described energising circuit. Relay AI operates further resulting in the transmission of more positive inductive impulses. As soon as relay AI has operated the circuit for relay XI is again interrupted at contacts IIaI or 48al and Bllhl. Relay XI restores again and interrupts the circuit for winding II of relay Al. The release of relay AI results in the interruption of the circuit over winding II of transformer Tr and hence in the transmission of a negative inductive impulse. As soon as relay AI has restored an energising circuit for winding II of relay XI is completed. Relay XI operates and again closes an energising circuit for winding II of relay AI. This interaction is repeated until the relay VI has restored. The release period of relay VI is arranged so that about three cycles of positive and negative impulses are transmitted. The pause between the impulses, i. e., the frequency of the impulses is arranged so that in all cases a positive inductive impulse occurs in the pause between the terminating signal impulses transmitted simultaneously in the opposite direction so that the release criterion is positively effective.

As soon as a positive release impulse has been received by the relay P2 in the incoming repeater Ue2 contact 90122 is closed. Since however relay M2 is energized over contact 90p2 in addition to the relay A2 the relay F2 is energised in the following circuit: earth, contacts 90112, 191222, winding I of relay F2, battery, earth. The relay F2 immediately interrupts the circuit for relay A2 at contact 18 2. After the operation of relay F2 the holding circuit for relay C2 is interrupted at contact 99 2. The relay F2 holds, until relay C2 has restored, in the following circuit: earth, winding II of relay F2, contacts IlIlIf2, I0'Ic2, resistance Wi2, battery, earth. The operation of the armature of relay P2 by a subsequent negative impulse and the holding of contact p2 thus efiected is in this case without effect since the relay F2 is maintained locally over its winding II. On the operation of relay F2 the circuit for the holding winding II of relay E2 is interrupted at contact I06f2. Relay E2 restores and on the one hand interrupts the accepting circuit for the relay P2 and on the other hand transmits guarding alternating current backwards over the junction. As soon as relay C2 has restored as already described the release of the subsequent selectors is initiated. After the release of relay C2 the circuit for the winding II of relay F2 is interrupted at contact I01c2 and the relay is only maintained over contacts 9102, 98e2 until the subsequently arranged group selector is restored and high resistance winding I of relay E2 has again been energised over the outgoing c-conductor. Thus the circuit for the winding II of relay F2 is interrupted at contact 98e2. On the release of relay C2 the relay M2 is restored by the short circuit over contacts 8Is2, 8502. Thus the repeater is ready to be seized again.

Release after called subscriber has hung up his receiver If the release is performed by the calling subscriber directly on the terminating signal transmission initiated by the hanging up of the receiver of the called subscriber the following operations occur:

When the called subscriber hangs up then in the final selector of the called subscriber earth is applied to the a-conductor and relay S2 of the repeater Ue2 is energized in the following circuit: earth on the a-conductor in the final selector, contacts 6902, 65122, winding I of relay S2, contact 66111.2, winding II of relay S2, contact H112, potential on the b-conductor in the final selector. Relay S2 operates and at contact 8Is2 interrupts the circuit for winding II of relay E2. Relay E2 restores and thus transmits an alternating current impulse backwards over the junction VL. By the release of relay E2 the following circuit is completed for relay V2: earth, contact H1402, winding II of relay V2, contacts 96e2, 9802, battery, earth. Relay V2 operates and at contact 65112 interrupts the circuit for relay S2 extending over the a-conductor. Relay S2 restores and at contact 8 Is2 again closes the circuit for relay E2. Relay E2 operates and terminates the alternating current impulse revertively transmitted. As soon as relay E2 operates the circuit for relay V2 is opened at contact 98e2. Relay V2 restores and at contact 65122 closes the circuit for relay S2. Relay S2 operates again and restores relay E2 whereby a further alternating current impulse is transmitted back over the junction. This interaction is repeated until the release impulse transmitted on the hanging up of the receiver by the calling subscriber is received by the repeater Uel. The first positive inductive impulse occurring in a pause between the terminating signal impulses effects the energising the relay P2. Relay P2 closes its contact 901:2 and thus connects the relay F2 over contact I9m2. Relay F2 holds over its winding II until the release of relay C2 and at contact l06f2 immediately and finally interrupts the circuit for relay E2, which applies guarding alternating potential back over the line. The relay P2 is set back to its normal position over its winding II after the operation of relay F2 in the following local circuit: earth, contacts 93u2, 9212, winding II of relay P2, battery, earth. After the release of relay C2 and operation of relay E2 over its winding I relay F2 finally restores. Relay M2 is short-circuited after the release of relay C2 over contact 8502 and also restores.

The transmission of the release inductive impulse is effected in the outgoing repeater Uel independently of the simultaneous terminating signals in the reverse direction, as soon as the calling subscriber has transmitted the signal for release by hanging up his receiver. Therefore as soon as the seizing relay CI has restored in the outgoing repeater Uel the relay AI and relay VI are immediately operated and the latter at contact 391;! opens a circuit maintained over contacts 38'uI, 35e| by means of relay J I for the receipt of the terminating signal and thus simultaneously closes the repeating circuit for the release impulse.

Release at the same time called subscriber answers The case will now be considered in which the reply impulse of the called subscriber occurs in the return direction in the outgoing repeater simultaneously with the release initiated by the calling subscriber. In this case also the transmission of the release impulse must be ensured since the terminating signal impulse of the called subscriber can occur directly on the connection of the reply impulse. The unfavourable moment is that in which after the initiation of the release of the outgoing repeater Uel relay Cl restores and relays AI and El are also energised. Since in this case relays YI and VI are not yet energised the accepting circuit for the incoming reply impulse is maintained over contacts 38vI, 36el so that the relay J I is energised. Since also relay E! is energised the energising circuit for relay YI is still interrupted at contact 42el. Relay J I at contact 2 IzI interrupts the circuit for relay AI which restores and over contact 2211 energises the relay HI. Relay I-II operates. In the meanwhile over earth, contacts 46ml, 48al, Mel, Mei, 4512!, winding II of relay BI, battery, earth, the relay BI is energised. Relay Bl over its contact 55M short circuits relay XI so that this is maintained in View of the delay until the reply impulse is completed. Relay BI whose energising circuit was opened after the operation of relay III is slow to-release due to the shortcircuit of the winding I over contact I Gel. During the energising of relays BI and HI relay YI is energised in the following circuit: earth, contacts 46m, IQhI, 53M, winding of relay Yl, resistance WiI, battery, earth. Over contact 28yl the relay VI also operates.

At contact Moi thecircuit of relay BI isin terrupted and this relay restores slowly and at I I contact 54b| completes the short circuitfor relayYl. As soon as the reply impulse is. completed the relay AI operates again and in series therewith relay HI is held. Now relay XI restores. XI already described, which serves to. produce the release impulse, now begins.

Figure 2 shows a variation of the principle of the invention which is obtained by slight alterations of the outgoing repeater Uel shown in Figure 1. 'Thus in place of the number 'of altera-;

tions of positive 'andnegative inductive impulses as release impulses after the reply of a called subscriber half waves of a low frequency alternating current, for example, a 25 cycle alternating current are transmitted.

For the unaltered parts of the the same references are used as in Figure 1. The alterations in the current path are indicated by dotted lines. In detail the method of working is as follows:

When the connection according to the arrangement in Figure 1 has been set upas far as the point at which the called subscriber replies, f

the calling subscriber then hangs up his receiver and the seizing relay CI in the repeated Uel restores, as already described. on the release of relay CI since relays XI, EI and YI are energised, in the first place relays AI and VI operate immediately and then later relay HI. Over contact 581M the alternating current source Q of 25 cycles per second is connected to the winding I of transformer Tr. Over winding II of the transformer Tr and contact 39M the alternating current is transmitted over the junction VL to the incoming repeater Ue2. As soon as the first positive half wave has been received by the 'accepting relay P2 the relay operates whereby as already described the release is initiated. Al-

ternating current Q remains connected to the junction line for sufficiently long for a positive half wave to get through the terminating signal impulse occurring simultaneously in the reverse direction in all cases.

The release period of relay VI is dependent on the operating period of relay BI, the release period of relay XI- and the release period of relay- Y. Moreover its own release period is extended by the short circuit of its winding. As soon as the seizing relay CI has restored relay BI operates in the following circuit: earth, contacts 45ml, 6011i, Mel, 4501, winding II of relay Bl, battery, earth. Relay XI which after the operation of relay HI is held energized in the following circuit: earth, contacts 46ml, Bfilyl, winding II of relay XI, battery, earth, is short-circuited after I 1. In a telephone system wherein talking connections are extended from a calling party to. a

called party over a trunk line which is open to direct current, means controlled by the calling party for releasing said connection either before The interaction between relays'AeI and.

repeater Uel or after the response of the called party, means effective to transmit a single inductive impulse over said trunk line to operate said release means before said response and means effective to transmit a train of successive inductive impulses over said trunk line to operate said release means after said response.

2. In a telephone system wherein talking connections are extended from calling to called parties over a trunk line, means controlled by the response of the called party for transmitting an impulse of alternating current over said trunk line, means controlled by the calling party for producing an induced impulse of long'duration and transmitting it over the trunk to release the connection before the response and means for transmitting a plurality of shorter successive inductive impulses over the trunk line thereafter to release the connection.

3. In a telephone system, a trunk line extending between exchanges and having only inductive connections with said exchanges, means for transmitting a single induced impulse over the trunk line to seize and guard the same, means for transmitting revertive impulses of alternating current in the reverse direction over the trunk line during the establishment of the connection, means for transmitting induced impulses of positive and negative polarity over the trunk line to release the connection.

4. A telephone system as claimed in claim 3 in which there is a polarized relay on the trunk line in the distant exchange which is operated by a positive induced impulse and released by a negative induced impulse.

EGON zmzow. HERBERT TOPFER. 

